1. Field of the Invention
The present invention relates to a method and apparatus for correcting frequency offset in a direct sequence spread spectrum communication system (DSS) and a storage medium storing a control program therefor, and is particularly applicable to this type of offset correction in a wide band code division multiple access (WCDMA) network such as the proposed universal mobile telecommunications system (UMTS).
The present application claims priority of British Patent Application No. 0023117.5 filed on Sep. 20, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
In cellular communication systems, the timing and frequency accuracy of transmissions from network base stations rely on very stable and highly accurate reference oscillators. As there are a fixed and relatively small number of network base stations in a system such as UMTS, or any other mobile phone system, the reference oscillators and the network base stations can be relatively expensive and accurate. An accuracy of e.g., 0.05 parts per million (ppm) is typical and more accurate oscillators are available. In such systems, however, there are typically far more mobile stations which communicate with the network base stations. In a system such as UMTS, these are mobile telephones which have to be sold at a competitive market price and therefore costs have to be minimized. Therefore, low cost reference oscillators such as voltage control crystal oscillators (VCXO) would usually be selected for the reference oscillators of mobile stations. The frequency accuracy of these low cost reference oscillators is relatively low, e.g., 5 ppm.
Because the accuracy of the mobile oscillators Is much less than that available to the base stations with their more accurate reference oscillators, significant problems can occur with synchronization between the base station transmission and the locally generated carrier frequency used for down conversion.